The present invention relates to a rubber-modified styrene resin composition which is excellent in sliding properties and which does not provide molded products having poor appearance during molding.
Considerable progress has recently been made in the technical developments in the fields of household appliances and office automation (OA) apparatuses and these apparatuses have been made more and more lighter and smaller accompanied by the increase in the degree of integration of large scale integrated circuits (LSI). Conventionally, mechanical parts of such apparatuses have been manufactured from metals, but there has been a remarkable tendency to attain weight saving thereof by making use of plastics as materials for, in particular, gears, bearings and chassises.
In such fields, there has been a need for plastics having excellent wear resistance, heat resistance and resistance to creep and conventionally, there has exclusively been used engineering plastics such as polyacetals and polyester resins.
Moreover, there has been a heed to save cost in addition to weight saving and miniaturization of such mechanical parts and the engineering plastics have gradually been replaced by less expensive plastics.
On the other hand, it is well known in such industrial fields that a resin composition having self-lubricating properties can be obtained by incorporating silicone oil into a styrene resin and it is already put into practical use in a certain number of applications, but such resin composition is still insufficient.
For instance, silicone oil-containing rubber-modified styrene type resins as disclosed in Japanese Patent Un-examined Publication (hereinafter referred to as "J.P. KOKAI") No. Sho 60-217254 are excellent in sliding properties, but cause various troubles during molding. More specifically, the amount of silicone oil to be incorporated into the styrene resin should be not less than 2 parts by weight to impart sliding properties required of such applications to the styrene type resins. The compatibility between silicone oil and the rubber-modified styrene resins is substantially low and silicone oil is liable to separate from the rubber-modified resins during molding as the added amount thereof increases. This leads to the production of molded products having poor appearance such as silver streaks and burn spots.
To solve this problem, it has been tried to elongate a molding cycle or to reduce the molding temperature. However, these methods cannot substantially solve the problem and thus it has been desired to solve the problem as soon as possible.
Furthermore, J.P. KOKAI No. Sho 59-108016 discloses a method for improving the dispersibility of silicone oil in a styrene polymer by adding silicone oil at a specified time in the course of polymerization when a styrene monomer is polymerized in the presence of a rubber substance, but this method cannot make it possible to solve the foregoing problem.
In addition, as disclosed in J.P. KOKAI Nos. Sho 62-39610 and Sho 63-182361, it is also conceivable that the aforementioned problem can be solved if the content of silicone oil in the styrene resins is relatively reduced by simultaneously using silicone oil and other sliding property-improving agents. However, the molded products resulting from such resin compositions are easily peeled off in thin layers at portions adjacent the mold gate and thus the problem has not yet completely been solved.